I used to be mechanical engineer for about 30 years and now I do translations for catalogues, leaflets, operation manuals, service manuals, etc …
It is long time I face the problem to translate documents with keeping the original layout and quality; there are always brand-logos, images, plans with inserted text, tables (technical data, specifications, troubleshooting …), certificates …
I ever did a 500 pages catalogue in Acrobat pro with … the TouchUp tool (yes, I did it).
I consider that for technical documentation the responsibility of the translator should extend to the final layout, in order to produce a perfectly intelligible document.
The pdf is designed as a printing-preparation tool and not an editor. An editing/layout tool has to be used.

By the actual state-of the art we have the following situations:

A. Large scale projects, multilingual targets, start from original editing formats (usually FrameMaker), managed by agencies; the flow is segmented to pros:
- text extraction in the CAT-tool,
- translation,
- reconstruction,
- final proof of the layout in pdf

B. Small to medium local jobs, from sources in Word or editing tools: the flow is similar, but shorter and concentrated.
Or if for different reasons the clients cannot provide the original editing format. They do their best to extract the text in Word, and ask for the text translation only, irrespective for the layout concerns. And that is all. I have to do the Word, sometimes a revision of the final pdf.

C. A good pdf document, with extractable text and graphics.
It has to be rebuilt in some editable format.

a. MSWord: unfortunately cannot be used.
This is a very developed flow-text editor, based on the type-writer pattern. His most valuable features are the spelling tool and the table tool.
But processing text with layout requirements, mixed with pictures is a mess, and the result is very huge files, difficult to manipulate.
I receive often from clients bad in-house attempts to transform a pdf in Word. I never use Trados directly on Word, but on ttx. The return from ttx in Word gives horrid results: sliding pictures and text boxes, damaged TOC, numbering patterns, indexes and headers or footnotes. I do what I can to get a final document to look as close to the source.

b. Use of a DTP-tool.
DTP tools have been developed by the logic of the printing technologies: object manipulation in layers, text boxes, management of colors and graphics, settings according to particular printing techniques.
I decided to use InDesign CS3 for the very good reason that SDL Trados 8 has a standard filter for inx format.

Procedure

1. Preparation of the source pdf (assumed a good one) in Adobe Acrobat Professional:
- Set a high resolution for snapshot images (160 – 200 p/i) Edit -> Preferences -> General Sometimes may be useful to do an image.pdf from which all text has been erased (with TouchUp tool)
- Extraction of images: Advanced-> Document processing-> Export all images
- Extraction of text: Export-> More formats-> Text (plain)

D. A bad pdf (snapshot or scan) or a scanned document in imaging formats.
MS Office Professional 2003 and 2007 have an excellent tool: MS Office Document Imaging; for advanced users, not installed as default, is to be installed custom.
The pdf or image can be transformed in tiff format. The MSDI has a powerful OCR facility. If lucky, you may have extracted a reasonable usable text. Then go to Cb. Procedure.

Conclusion
It is a very safe procedure that ensures a layout and a graphic quality similar to the original, but with expensive resource consumption.
For some clients and/or similar product ranges (power-tools e.g.) productivity may be increased by defining enhanced templates with typical structure and contents, paragraph styles and print settingsThe critical point is that this really doubles the costs: the time needed to layout a page is equivalent to the translation of the 200 – 250 words contented.
And you have to consider the soft value and the hard needed: a powerful workstation, with a good graphic card and 2 monitors.